1. Field of the Invention
The present invention relates to an electric power steering apparatus of a rack-and-pinion type.
2. Description of Related Art
In a steering apparatus of a rack-and-pinion type that applies a manual operation force produced by operating a steering member to a rack shaft through a pinion shaft, there is an electric power steering apparatus that applies to a rack shaft a steering assist force produced by an electric motor.
There is provided an electric power steering apparatus of a so-called rack assist type comprising a speed reduction mechanism including a first bevel gear attached to an output shaft in an electric motor for steering assist and a second bevel gear arranged around a rack shaft and meshed with the first bevel gear and so adapted that the rotation of the second bevel gear is converted into the axial movement of the rack shaft through a ball screw mechanism (see Japanese Unexamined Patent Publication (KOKAI) No. 9-66844A (1997), for example).
In the above-mentioned electric power steering apparatus, a rotating force applied to the rack shaft from a pinion shaft and a rotating force applied to the rack shaft from a ball screw are exerted in opposite directions in such a manner that the direction in which a ball screw groove is twisted with respect to a ball screw shaft and the direction in which rack teeth are twisted with respect to the rack shaft are the same.
The output shaft in the electric motor is rotated in a first or second direction of rotation depending on the direction of operation of a steering member. The first bevel gear is moved in the direction in which backlash is decreased by a driving reaction force received by the first bevel gear from the second bevel gear when the output shaft is rotated in the first direction of rotation, while being moved in the direction in which the backlash is increased by the driving reaction force when the output shaft is rotated in the second direction of rotation.
On the other hand, the second bevel gear may, in some cases, be moved in the direction in which the backlash is decreased by a driving reaction force received by a ball nut from a screw shaft in the rack shaft when the output shaft is rotated in the first direction of rotation, while being moved in the direction in which the backlash is increased by the driving reaction force when the output shaft is rotated in the second direction of rotation.
In this case, the backlash becomes approximately zero when the output shaft is rotated in the first direction of rotation while the amount of increase in the backlash is increased when the output shaft is rotated in the second direction of rotation, so that mesh between both the bevel gears is violently changed (shifted). The shift in the mesh between both the bevel gears depending on the direction of rotation exerts a great effect on the gears. Particularly, the backlash becomes approximately zero, so that smoothness in the mesh between both the bevel gears is lost, and durability in the mesh is reduced.
Generally, it is preferable that the bevel gear uses not straight teeth but oblique teeth or curved teeth in order to ensure its strength and smoothness in the mesh. If diagonal teeth or curved teeth are used, however, the above-mentioned problem is easily exposed depending on the setting of an angle at which the axes of both the bevel gears cross each other and a speed reduction ratio therebetween.
An object of the present invention is to provide an electric power steering apparatus capable of always ensuring proper backlash between gears for transmission which are meshed with each other and capable of improving smoothness and durability in the mesh.